Malignant Mesothelioma (MM) is a relatively common, rapidly progressive and treatment-resistant malignancy that is associated with exposure to asbestos. Both human patients and mouse models of MM show frequent loss of the neurofibromatosis type II (NF2) gene, and re-expression of NF2 cDNA in Nf2-/- MM cells slows their proliferation and restrains their motility, implying that NF2 plays a causal role in MM. The protein product of the NF2 gene, Merlin, binds to and inhibits p21-activated protein kinases (Paks), enzymes that positively regulate cell cycle progression, survival, and motility. Recent evidence from our laboratories strongly suggests that Pak is required for transformation in cells lacking the NF2 gene. We postulate that loss of Pak function will lead to diminished activation of key Merlin effector pathways in NF2-deficient MM cells as well, and thus could benefit patients with MM. We propose two aims: 1) Using pharmacologic and genetic means to disable Pak function, we will establish the signaling pathways affected by Pak in MM cells, and also identify the key substrates of Paks in these cells that affect cell survival; 2) We will cross Nf2f/f; Ink4a/Arff/f mice, which develop MM upon intrapleural injection of Adeno-Cre, with a transgenic mouse that conditionally expresses a specific Pak inhibitor. This experiment will allow us to determine if loss of Pak function affects MM incidence and/or progression, as well as establish the in vivo signaling pathways that mediate this effect. The proposed studies will not only increase our understanding of cardinal signaling pathways, but could establish Paks as suitable targets for therapeutic intervention in this otherwise untreatable disease.